1. Field of the Invention
The present invention relates to a method of manufacturing a disc, and more particularly, to a method of and an apparatus for forming a transparent layer on a disc substrate, and a disc therefrom.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a convention optical disc. Referring to FIG. 1, the disc includes a disc substrate 10, a transparent layer 15, and an axial hole 14 formed at a center of the disc. In a case of a high density digital versatile disc (HD-DVD), a thickness T of the disc substrate 10 is 1.1 mm, and a thickness “d” of the transparent layer 15 is 0.1 mm, and thus a total thickness of the optical disc is 1.2 mm. In particular, in order to precisely focus a laser beam, which is radiated from a reproducing apparatus, on a surface of the transparent layer 15, the thickness “d” of the transparent layer 15 should be within a range of 100 μm±3 μm. Methods of forming the thin transparent layer 15 include a spin coating method.
FIG. 2 is a reference view illustrating a conventional spin coating method of forming a transparent layer 15 on a substrate 10. In the conventional spin coating method, a liquid ultraviolet hardening resin 13 is dropped at a point of a surface of the disc substrate 10, which rotates at a low speed, as shown in FIG. 2, and then, the disc substrate 10 is rotated at a high speed. The liquid ultraviolet hardening resin 13 is spread out along the surface of the disc substrate 10 in a radial direction by a centrifugal force to coat the disc substrate 10. Next, ultraviolet light is radiated onto the coated ultraviolet hardening resin 13, thereby hardening the ultraviolet hardening resin 13.
When forming the thin transparent layer 15 by the spin coating method, the thickness “d” of the transparent layer 15 depends on processing conditions such as a spinning speed, a spinning time, a dispensing position of the ultraviolet hardening resin 13, and a dispensing amount, and a property of the resin 13, such as viscosity. In particular, in order to coat the transparent layer 15 to the thickness T of 100 μm, the viscosity of the ultraviolet hardening resin 13 should be more than several centi-poise (cps). As the viscosity of the resin 13 increases, uniformity of the thickness “d” of the transparent layer 15 increases, and simultaneously, a molecular attraction increases, thereby affecting the discharged position of the ultraviolet hardening resin 13 on the disc substrate 10.
FIG. 3 is a graph illustrating a relationship between the dispensing position of a resin and the uniformity of the thickness of a layer. FIG. 3 presents the uniformity of the thickness of the transparent layer 15, which is generated when the ultraviolet hardening resin 13 is dispensed at the axial hole (center) and at 20 mm, 30 mm, 40 mm, and 45 mm of a radius, respectively, of the disc substrate 10 under conditions of the viscosity of 5000 cps, the dispensing amount of 5 g, and the spinning speed of 700 rpm. As shown in FIG. 3, the thickness of the transparent layer 15 is most uniform when the ultraviolet hardening resin 13 is dispensed at the center (axial hole 14) of the disc substrate 10.
However, as mentioned above, in the conventional spin coating method, the ultraviolet hardening resin is not dispensed at a center of the disc substrate but at a point of the surface of the disc substrate.